CA1167284A

CA1167284A - Belt cylinder equipment

Info

Publication number: CA1167284A
Application number: CA000376647A
Authority: CA
Inventors: Oskar Peter
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-04-30
Filing date: 1981-04-30
Publication date: 1984-05-15
Also published as: DE3016696C2; ZA812823B; SU1102492A3; JPS5740107A; US4472981A; DE3016696A1

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to belt cylinders, i.e. piston-rodless operating cylinders in which steel belts, in particular, are used in transferring force; even during production, belts of this kind have the negative property of not being suffici-ently straight, instead they are bowed and this leads to break-downs in that the belts either destroy the flanges on the deflecting rollers in a very short time or, if there are no flanges, they run off the rollers; for this reason in spite of the technical advantages, belt cylinders have not been successful in practice; satisfactory operation of these belt cylinders may be achieved if, and only if, the belts which transfer force from the piston to the force pick-up are in absolute alignment; in order to achieve this, the invention provides that the piston - belt - force pick-up - piston system be pre-loaded, and that the ends of the belt be secured pivotably in a turnbuckle, so that they may automatically align themselves; moreover, the relationships between diameter, deflecting rollers and belt dimensions are optimized according to the driving force, from the point of view of fatigue strength.

Description

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~ he invention relates to a so-called belt cylinder comprising a fluid driven piston, in which the force is trans-ferred by a belt, preferably a steel belt, running over rollers at each end of the cylinder and moving a force pick-up outside the cylinder.
It has been found that belt cylinders of this kind, kn~wnfor example from German Patent 1,293,037 and British Patent 1,192,249 and also known as piston-rodless operating cylinders, have certain disadvantages in the form of operating deficiencies and for this reason have hitherto been unsuccessful in practice.
One of the more serious disadvantages of these belt cylinders is the guidance of the belt upon the deflecting roller or rollers which must usually be fitted with flanges to prevent the belt from drifting sideways as a result of, among other things, production-related non-linearity of the belt, the latter being often bowed and therefore failing to align itself. Breakdowns occur, especially over long cylindex lengths, leading to the destn~ion of the belt, the rollers, or both,~ since the belt runs against or onto the flanges. It has been found practically impossible to guide the steel belt on the deflecting rollers.
The result of this was constant breakdowns of the belt cylin-ders, Furthermore, the assembly of these units required a maximum of accuracy, since the whole system must be in absolute alignment vertically and horizontally. Rigid attachment of the belt is difficult per se and is rendered even more difficult by any bow in the belt. Moreover tensile and compressive loads in the belt, as it is guided over the deflecting rollers, also ` have a negative ef~ect.
If a belt is to be guided over deflecting rollers, it 3~ is essential for the attachment of the belt in or to the piston and the f~rce pick-up to be in absolute alignment and this is :: -- 1 --extremely difficult to achieve because of the bow in the belt, especially if the deflecting rollers are at some distance from each other. For satisfactory operation, however, it is also essential for the piston, the force pick-up, the deflecting rollers and the belt or belts to be in alignment. This has so far been impossible.
Even if the belts are made of high-grade material, for example chrome steels, loads acting upon them result in permanent deformation. Additional stresses arise from flutter, which is usually impossible to avoid at high piston speeds.
The result is premature breakage of the belts. For all of these reasons, belt cylinders of this kind, in spite of their advan-tages, have been unable in practice to compete with conventional operating cylinders.
On the other hand, a sufficiently thin steel belt, as compared with a cable, has the advantage of being able to run over much smaller deflecting rollers, and is therefore particu-larly suitable for the transfer of force. Furthermore, in the case of steel belts, thera are not sealing problems, and this is another reason for using them. If the smallest possible deflecting rollers are used, the steel belt may run close to the outside of the operating cylinder. This means that the cylinder ;~ and cylinder heads may be smaller and the belt may be suspended along the centreline of the piston without increasing the size of the heads. These are design principles which cannot be achieved with a cable.
In spite of the use of thin belts, tensile and com-pressive stresses arise therein as they pass around the rollers at the ends of the cylinder.
The following load cases may arise in the cross-sec-tion of the belt:

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1. in the loaded half of the belt:
a) during deflection over the rollers, the sum of the load-ing, tensile and compressive stresses arising from the deflection;
b) outside o-E the deflecting rollers, only loading stresses arise;
2. in the unloaded half of the belt:
a) in the deflecting roller, tensile and compressive stresses arising from deflection of the belt;
b) outside the deflecting roller: freedom from stressj In an operating cylinder of this kind, it is impossible to overcome tensile stresses with extremely wide and thin belts, unless larger dimensions are accepted, but this greatly reduces the ad~antages of such a unit and therefore restricts its field of application~
An attempt might also be made, for example, to achieve : a life comparable to that obtained with piston-rod cylinders, merely by adequate enlargement of the diameter of the rollers, but this results in a sharp increase in overall dimensions, and in many ca9es, for instance busdoors, etc., there is not enough room for this. If, as has already been suggested, cables were used to tranefer the force instead of belts, this would present still more space problems, since the rollers would have to be ; ~ larger in diameter than those used for belts, in order to avoid premature wear in cables passing around rollers.
~ ow various fatigue test have shown that the diameter of the deflecting rollers must be as small as possible, but only small enough for the maximal tensile and compressive stresses arieing in the unloaded belt to be less than the :30 fatigue strength of the belt under reverse bending loads; and, on the other hand, that..the width of the belt for a given ~:

.:

thickness must be such that the load imparted by the driving element (the piston) produces', at the most, a stress corresponding to the difference between the fatigue strength under pulsating tensile stress and the reverse bending load.
It is the purpose of the invention to eliminate these disadvantages of belt cylinders and to provide a unit of this kind', the belt or belts of which are controllable between their attachment and suspension points and will run permanently in alignment. over the deflecting rollers, the said unit having a long life', and the belt cylinder', especially the cylinder heads', being small in dimension as compared with the inside diameter of the cylinder.
A construction in accordance with the present invention comprises a belt cylinder including a cylinder', a fluid driven piston disposed within the cylinder and a pair of rollers without flanges located at each end of the cylinder. A force pick-up is disposed outside the cylinder and a flat belt having ends pivotally connected to the force pick-up member runs from the pick-up member to the piston over the rollers. The belt is tensioned to at least 25% of the ma~imum driving force of the piston.
According to a more specific version of the nvention, this purpose is achieved in that the system pre-load', with exclusive use of the belt as the spring element, amounts to at least 50% of the possible maximal ~drlving force of the piston', whereas the said system pre-load', with the use of at least one spring arranged in the force pick-up in order to apply the pre-load, amounts to up to about 25% of the maximal driving force of the said piston', the:ends of the said belt being mounted pivotably on the said r~
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piston and/or on the said force pick-up, and the deflecting rollers having no flanges.
According to the invention, the necessary tension is applied by the pre-loaded belt or belts.
According to the invention, the necessary tension may be applied, as an alternative, by tension springs arranged between the belt or belts.
According to one advantageous embodlment of the invention, the ends of the belt are suspended at the force pick-up in a turnbuckle with pins in sliding parts held at variable distances apart by means of adjustable stops, the said sliding ~ i'7~

parts being arranged, by means of pins, in alignment in the plane of the belt, and at least one tensîon spring being provided between the sliding parts with pins.
According to the invention, the system pre-load, with exclusive use of the belt as the spring-element, amounts to at least 50% of the possible maximal driving force of the piston, and up to about 25% of the maximal driving force of the piston if the pre-load is produced by spring force.
The ends of the belt are pref~rably provided with reinforcements.
According to the invention, the pins may also be replaced by a ball and socket joint.
The use in belt cylinders of the device according to the invention allows the belt to adjust itself automatically as it moves back and forth, i.e. to align itself. Any irregulari-ties in the system, especially as regards alignment of the belt between its suspension points, are thus compensated for. A belt under tension loses most of its bow.
The belt thus no longer runs against or over th~
flanges of the deflecting rollers, nor does it run sideways off the rollers if no flanges are provided.
As an element for the transfer of force, the belt experiences in the load case, i.e. when the piston is acted upon by compressed air and this force is transferred by the .
belt to the outside by means of the force pick-up, a load from ~ the said force. As a result of this, the belt lengthens within ;~ the resilient range of the material of which it is made.
; Slackening of the belt on the unloaded side, resulting in mis-alignment, is eliminated by the invention in that any lengthen-ing of the belt on the loaded side is always compensated for, on the unloaded side, either by reducing the pre-load by the _ 5 --amount of the said lenythening or by contraction of one or more springs in the turnbuckle.
If the belt is used in the system both as a force-transfer element and, conceivahly, as a spring element, 50%
of the maximal possible driving force is needed as pre-loaa since, with a smaller pre-load, the unloaded side of the belt cannot compensate for the lengthening. A simpler alternative is to produce the pre-load by one or more springs to compen-sate for lengthening. In this case, the pre-load re~uired is relatively small since it is added to the force produced by the piston.
According to the invention, the load on the belt is made up of the reverse bending load (0.7 ~ 1.25) . a, the driving element load (piston and cylinder) of about (0.7 - 1.25) . c, and the load imposed by fatigue under pulsating tensile stress, a maximum of (0.7 ~ 1.25) . b, ~herein:

a = = fatigue strength under reverse bending stress D bw (stress deflection for 2.3% failure probability and at least 10 load cyclesj;

b = = fatigue strength under pulsating tensile stress Dzsch (maximal stress for 2.3% failure probability and at least 10 load cycles with minimal stress = 0), c =
Dzsch - Dbw for the belt material selected.
As a result of this, optimal use is made of the belt under the types of load arising and, at the same time, belt life is almost unlimited.
It has been found desirable to use high grade chrome steels for the belts.

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Belts of sandwich design may also be used for the transfer of force.
Belts of this kind may be in the form of sandwiches of steel-plastic-steel, steel-adhesive-steel, steel-rubber-steel, or the like. This can also save space, since the sandwich design permits the use of narrower belts.
Finally, it is desirable to attach the belt to the piston eccentrically. This has the advantage of shifting the axes of the rollers by an equal amount towards the axis of the cylinder and thus results in smaller cylinder heads.
It has been found that belt widths of between 1/3 and 2/3 of the inside diameter of the cylinder are particularly satisfactoryO
According to the invention, the following dimensions may be obtained by making the belt of a high-grade chromium steel, for example, having a fatigue strength (107 load cycles, 2.3% failure probability) under a pulsating tensile stress of 1150 N/mm2, and a reverse bending stress of + 750 ~/mm :
al in the case bf a belt cylinder of 40 mm inside diameter for an operating pressure of max. 10 bars:
belt dimensions: width 20 mm, thickness 0.15 mm roller diameter: 55 mm ~;~ belt attachment to piston: 10 mm off centre;
b) in the case of a belt cylinder of 80 mm inside diameter for an operating pressure of max. 10 bars:
belt dimensions: width 50 mm, thickness 0.25 mm roller diameter: - 90 mm belt attachment to piston: 20 mm off centre.
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The invention is illustrated in particular and pre-ferred embodiments by reference to the accompanying drawings in which:
FIGURE 1 shows a belt cylinder, FIGURE 2 is a side elevation of detail A in Fi gure l;
FIGURE 3 is a plan view of Figure 2 in partial section, FIGURE 4 shows an embodiment of the belt cylinder with no spring, , FI GURE 5 is a diagram.
The belt cylinder illustrated in Figure 1 consists essentially of a cylinder 1, a piston 2, cylinder heads 3, 4 deflecting rollers 5, 6 a belt 7 attached to piston 2, running over the deflecting rollers 5, 6 and carrying an external force pick-up 8 which may be integral with a guide bush 9~
The transfer of force is effected by piston 2 through belt 7 : and stops 22, 23 to force pick-up 8.
: Belt 7 may either be in one piece, in which case it passes uninterruptedly through piston 2, or it may be in two pieces suspended from the left and right hand ends of the piston 2. The belt 7-is in any case divided at force pick-up 8 and is reinforced with strips, tabs, or the like lla, llb which provide the necessary reinforcement around openings in the ends cf the belt 7. Ends 7a, 7b of the belt 7 are sus-pended, within turnbuckle 10, in the vicinity of force pick-up B, from pins 12, 13 at each end of the belt 7, the pins, ~ - 12, 13 being parts of sliding pieces. In the embodiment in : Figure 1 the two sliding pieces are divided by a drive element 14 and are braced in relation to each other by means of springs 15, 16, spring 15 being hooked to pin 17 in sliding piece 12 and to pln 18 is sliding piece 13, while spring 16 is hooked to corresponding pins 19, 20. Sliding pieces 12, 13 and drive element 1~ comprise, in addition to holes for springs 15, 16 a central hole for a threaded pin or the like 21 provided on each side with a stop 22, 23 for the said sliding pieces 12, 13. This makes it possible for the belt 7 to be set to stops 22,23 for the transfer of force, in order to compensate for production and length tolerances.
Figure 4 shows a belt cylinder of the invention without springs 15, 16 the pre-load being provided merely by belt 7. Here again, ends 7a, 7b of the belt 7 have open-ings reinforced with straps, tabs or the like lla, llb, from which the sliding pieces 12, 13 are suspended by means of pins. In this case, stops 24,25 associated with sliding pieces 12, 13 are mounted adjustably in blocks 26, 27 rigidly secured to the ends of turnbuckle 10.
Ends 7a, 7b of the belt 7, suspended from the pins in the sliding pieces 12, 13 may pivot about these pins and are braced in relation to each other. It has been found that the system pre-load must be at least 50% of the maximal possible force applied by piston 2, if no springs 15, 16 are provided. On the other hand, springs are provided, which makes for simpler assembly, they must be pre-loaded to a maximum of 25% of the piston force. Belt cylinders of this design operate with no breakdowns for long periods of time, since problems arising from misalignment of the belt 7 are .
; eliminated. The belt 7 is no longer bowed and remains in correct alignment with the connections at force pick-up 8, with deflecting rollers 5, 6, and with piston 2 in cylinder 1.
The belt 7 does not ride up onto the flanges of the deflect-ing rollers nor, if there are no flanges, does it run off .

_ the rollers. Another advantage achieved with the invention is that assembly of the belt cylinder is ~uicker and simpler because the belt 7 is fitted in one piece. On the inside of the belt 7, the stress varies between compression according to load cases A and C in Figure 5, freedom from stress, and stress from cylinder loading, i.e., reverse bending stress is present. The durability of the inside of the belt is thus determined by its fatigue strength under reverse bending stress~ If the device is to be adequately durable, the fatigue strength of the belt must provide 97.7%
safety against failure under 10 load cycles. In load case A, for example, this is exceeded and fatigue strength is therefore inadequate. On the outside of the belt, the stress varies between no stress at all and tensile stress from the deflection and loading, i.e., there is fatigue arising from pulsating tensile stress. The durability of the outside of the belt is thus determined by fatigue strength under pulsating tensile stress. In load case D, for example, this is exceeded. Again fatigue strength would be inadequate.

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Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A belt cylinder comprising:
a cylinder, a fluid driven piston disposed within said cylinder;
a pair of rollers without flanges located at each end of said cylinder;
a force pick-up disposed outside said cylinder; and a flat belt having ends pivotally connected to said force pick-up member and running from said pick-up member to said piston over said rollers, said belt being tensioned to at least 25% of the maximum driving force of the piston.

2. A belt cylinder according to claim 1, wherein said belt is a steel belt.

3. A belt cylinder according to claim 1, wherein the pre-loaded belt is adapted to apply the necessary tension.

4. A belt cylinder according to claim 1, including tension springs provided between the ends of the belt adapted to apply the necessary tension.

5. A belt cylinder according to claim 1, wherein the ends of the belt are held to the force pick-up in a turn-buckle at variable distances apart, by means of adjustable stops, in sliding pieces with pins in alignment in the plane of the belt, at least one tension spring being provided between said sliding pieces.

6. A belt cylinder according to claim 4, wherein the ends of the belt are held to the force pick-up, in a turnbuckle at variable distances apart by means of adjust-able stops, in sliding pieces with pins in alignment in the plane of the belt, at least one tension spring being provided between said sliding pieces.

7. A belt cylinder according to claim 5 or 6, wherein the sliding pieces are provided with ball and socket joints.

8. A belt-cylinder according to claim 1, 2 or 3, wherein the load on the belt is made up of a reverse bending load () . a, the driving element load (cylinder and piston) of about () . c, and the load imposed by fatigue under pulsating tensile stress, a maximum of () . b, in which:

a = = i.e. repeated loading under reverse-bending Dbw stress (stress deflection for 2.3% failure probability and at least 106 load cycles);
b = = i.e. fatigue strength under pulsating tensile Dzsch stress (maximal stress for 2.3% failure probability and at least 106 load cycles with minimal stress = 0);
c =
Dzsch - Dbw for the belt material selected.

9. A belt cylinder according to claim 1, 2 or 3, wherein the belt is attached to the piston slightly off-centre in a direction opposite to its circulation.

10. A belt cylinder according to claim 1, 2 or 3, wherein the belt has a width amounting to one third to two thirds of the inside diameter of the cylinder.

11. A belt cylinder according to claim 1, 2 or 3, wherein the belt is of sandwich construction.